The latest Fedora development snapshot makes Ext4 the default file system and adds experimental support for the next generation btrfs filesystem. "According to current plans, version 11 of Fedora, which is expected to arrive in late May, will use Ext4 as its standard file system. That's what the Fedora Engineering Steering Committee (FESCo) recently decided, following a heated discussion in an IRC meeting. If however Ext3's successor encounters big problems with the pre-release versions of Fedora 11, the developers will dump that plan and revert to Ext3."

Ext4 wasn't "unreleased". It was marked as a development filesystem and has been merged in the Linux kernel with that tag a while back. In Fedora 9, you have to pass a explicit boot option and was clearly noted as experimental so only those who understand the risks will be involved in early testing.

Red Hat has a few full time kernel developers working on Ext4 and btrfs so I am sure they understand what is involved and can get a headstart.